The present invention relates to an active matrix type liquid crystal display apparatus for use in a liquid crystal television set, a notebook personal computer, and the like.
FIGS. 24 and 25 are a plan view and a sectional view, respectively, of a conventional active matrix type liquid crystal display apparatus. The active matrix type liquid crystal display apparatus is constituted essentially of a liquid crystal panel 1, a gate driver 2, a source driver 3, and a backlight 4.
The liquid crystal panel 1 has an active matrix board 5, an opposed board 6, a liquid crystal layer 7 sandwiched between the active matrix board 5 and the opposed board 6, and a polarizer (not shown) attached to the outer side of each of the active matrix board 5 and the opposed board 6.
On an insulation substrate 5a of the active matrix board 5, there are provided a plurality of scanning lines (not shown) disposed parallel with one another, a plurality of signal lines 9 parallel with one another and orthogonal to the scanning lines with an insulation film 8 disposed between the signal lines and the scanning lines, thin film transistors (TFTs) 10 disposed in the vicinity of intersections of the scanning lines and the signal lines 9, and a plurality of pixel electrodes 11 disposed in regions surrounded with the scanning lines and the signal lines 9.
FIG. 26 is a plan view showing a one-pixel part of the active matrix board 5. Because the pixel electrode 11 and the signal line 9 are formed in the same layer, the pixel electrode 11 is spaced at a predetermined interval from the signal line 9 to prevent the pixel electrode 11 from contacting the signal line 9. In the TFT 10 which is a three-terminal element, electrical continuity between a drain electrode 13 and a source electrode 14 is controlled by a voltage applied to a gate electrode 12. The gate electrode 12 is connected to a scanning line 15 adjacent thereto. The source electrode 14 is connected to the signal line 9 adjacent thereto. The drain electrode 13 is connected to the pixel electrode 11.
The opposed board 6 is provided with color filters 16 formed in the order of red, green, and blue at positions corresponding to each pixel electrode 11. A black matrix 17 is formed between the adjacent color filters 16 and 16. The black matrix 17 serves as a light shield film for preventing leak of light from the gap between the pixel electrode 11 and the scanning line 15 as well as the signal line 9. An opposed electrode 18 made of a transparent conductive material is formed on a layer of the black matrix 17 and the color filters 16. The gate driver 2 and the source driver 3 are connected to terminals of the scanning lines 15 and those of the signal lines 9, respectively, disposed on the periphery of the liquid crystal panel 1.
The method of driving the active matrix type liquid crystal display apparatus having the construction will be described below.
When writing to an array of pixels of an nth row, an ON-signal (electric potential Vgh at which the TFT 10 is turned on) is input to a scanning line 15n of the nth row from the gate driver 2. At this time, an OFF-signal (electric potential Vgl at which the TFT 10 is turned off) is input to scanning lines other than the scanning line 15n. Thus, only the TFTs 10 of the nth row are turned on. On the other hand, source signals having voltages to be applied to the pixels (pixel electrodes 11 and liquid crystal layer 7) of the nth row are supplied to each signal line 9 from the source driver 3.
Upon completion of write for the array of the pixels of the nth row terminates, the OFF-signal is input to the scanning line 15n, whereas the ON-signal is input to the next scanning line 15(n+1). All pixels are charged with voltages corresponding to data by repeating the operation. The transmissivity of the liquid crystal layer 7 disposed between the pixel electrode 11 and the opposed electrode 18 changes depending to a voltage applied across the pixel electrode 11 and the opposed electrode 18, and light emitted from the backlight 4 is therefore adjusted. As a result, images are displayed on the active matrix type liquid crystal display apparatus.
There is proposed a construction in which pixel electrodes are provided on an interlaminar insulation film so that the pixel electrodes and the signal line are formed as different layers and that the pixel electrodes overlap the signal lines (disclosed in Japanese Patent Application Laid-Open No. 63-279228). FIG. 27 is a sectional view showing a one-pixel part of an active matrix type liquid crystal display apparatus having the above-mentioned construction in which pixel electrodes overlap signal lines. FIG. 28 is a plan view of an active matrix board 24 shown in FIG. 27. In the construction, pixel electrodes 21 and signal lines 22 are formed as separate layers, and the pixel electrodes 21 are overlaid on the signal lines 22 through an interlaminar insulation film 23. Thus, it is possible to eliminate the gaps between the pixel electrodes 21 and the adjacent signal lines 22. Thus, it is possible to enlarge the area of the pixel electrodes 21 (aperture ratio) and thus reduce the power consumption of the active matrix type liquid crystal display apparatus. In FIGS. 27 and 28, reference numeral 24a denotes an insulation substrate, 25 denotes a TFT, 26 denotes a liquid crystal layer, 27 denotes an opposed electrode, 28 denotes an opposed board, 29 denotes a scanning line, 30 denotes a contact hole, 31 denotes an auxiliary capacitor electrode, and 32 denotes an auxiliary capacitor line.
However, in comparison with the construction shown in FIG. 26 in which the pixel electrode 11 is spaced at a predetermined interval from the signal line 9, the construction in which the pixel electrodes 21 overlap the signal lines 22 invites an increased capacitance Csd between the pixel electrode 21 and the signal line 22. With the increase of the capacitance Csd, the source signal causes a pixel electric potential to change easily. Eventually, there will occur display characteristic deterioration called shadowing phenomenon.
The mechanism of the shadowing phenomenon will be described below by using an equivalent circuit of the active matrix board 24 shown in FIG. 29. When a TFT 25 is turned on as a result of input of an ON-signal Vgh to a scanning line Gn, a pixel electrode P1 is supplied with a voltage Vs1 from a signal line S1.
Next, when the TFT 25 is turned off as a result of input of an OFF-signal Vgl to the scanning line Gn, a voltage Vs1xe2x80x2 corresponding to data to be written to a pixel electrode P2 of a next stage is supplied to the signal line S1. At this time, the voltage of the pixel electrode P1 is influenced by the voltage Vs1xe2x80x2 of the signal line S1 through the capacitance Csd1. Supposing that the voltage of the pixel electrode P1 at that time is Vp1, the voltage Vp1 is expressed as follows:
Vp11=Vs1xe2x88x92(Csd1(Vs1xe2x88x92Vs1xe2x80x2)+Csd2(Vs2xe2x88x92Vs2xe2x80x2))/(Cp+Csd1+Csd2)xe2x80x83xe2x80x83(1)
where Cp is a capacitance of the pixel electrode (Cp=liquid crystal capacitance, Clc+auxiliary electrode capacitance, Ccs), Csd1 is a capacitance between the signal line S1 and the pixel electrode P1, Csd2 is a capacitance between a signal line S2 and the pixel electrode P1, Vs1 and Vs2 are voltages of the signal lines S1 and S2, respectively, in the case where the scanning line Gn of an nth row is in an ON-state, and Vs1xe2x80x2 and Vs2xe2x80x2 are voltages of the signal lines S1 and S2, respectively, in the case where a scanning line G(n+1) of an (n+1)th row is in an ON-state.
In a gate line inversion driving method (namely, xe2x80x9c1H inversion drivingxe2x80x9d) which is a conventional method of driving the active matrix type liquid crystal display apparatus, the polarity of the source signal is inverted every line of gates. Supposing that adjacent gradations are the same,
Vs=Vs1=Vs2, Vsxe2x80x2=Vs1xe2x80x2=Vs2xe2x80x2xe2x80x83xe2x80x83(2)
Therefore, from the equations (1) and (2),
Vp1=Vsxe2x88x92(Csd1+Csd2)/(Cp+Csd1+Csd2)xc2x7(Vsxe2x88x92Vsxe2x80x2)xe2x80x83xe2x80x83(3)
As is obvious from the above, in the 1H inversion driving, the amount of change of the pixel electric potential is proportional to (Csd1+Csd2). Therefore, with the increase of the capacitance Csd between the signal line S and the pixel electrode P, the shadowing phenomenon appears conspicuously.
A dot inversion driving method has been proposed as a driving method suppressing the change of the pixel electric potential due to the capacitance Csd between the signal line S and the pixel electrode P. In the dot inversion driving, the polarity of the source signal is inverted not only every line of the gates, but also every line of sources.
Supposing that adjacent gradations are the same in the dot inversion driving,
Vs=Vs1=xe2x88x92Vs2, Vsxe2x80x2=Vs1xe2x80x2=xe2x88x92Vs2xe2x80x2xe2x80x83xe2x80x83(4)
From the equations (1) and (4),
Vp1=Vsxe2x88x92(Csd1xe2x88x92Csd2)/(Cp+Csd1+Csd2)xc2x7(Vsxe2x88x92Vsxe2x80x2)xe2x80x83xe2x80x83(5)
From the above, in the dot inversion driving, the variation of the pixel electric potential is proportional to the difference between the capacitance Csd1 and the capacitance Csd2. Therefore, the dot inversion driving is much superior to the 1H inversion drive in suppressing the occurrence of the shadowing phenomenon. Thus, the dot inversion driving can improve the image quality of the liquid crystal display apparatus. In particular, by reducing the difference between the capacitances Csd1 and Csd2 in connection to the pixels adjoining in the direction in which the scanning line 29 extends, it is possible to suppress the occurrence of the shadowing phenomenon to a great extent.
However, the following new problem occurs. In general, in producing a liquid crystal display apparatuses, a photolithographic process is performed block by block. Thus, an alignment deviation occurs from block to block. This leads to the variation in the amount of overlapping between the pixel electrode P and the signal line S and hence the variation in the capacitance Csd between the signal line S and the pixel electrode P. In the case where the dot inversion driving is adopted, the pixel electric potential is liable to change due to the variation in the capacitance Csd. This results in difference of transmissivity among the blocks.
For example, referring to FIG. 30, let it be supposed that an alignment deviation dx has occurred in the photolithographic process of the pixel electrodes P. In this case, there is an increase in the amount of overlapping between the pixel electrode P and the signal line S1. Thus, there is an increase in the capacitance Csd1 between the signal line S1 and the pixel electrode P, whereas there is a decrease in the capacitance Csd2 between the signal line S2 and the pixel electrode P. FIG. 31 shows the relationship between the alignment deviation dx in the photolithographic process and the capacitances Csd1, Csd2. FIG. 31 indicates that with the increase of the alignment deviation dx, the difference between the capacitance Csd1 and the capacitance Csd2 becomes big, and the amount of variation of the pixel electric potential increases.
In the ordinary conventional photolithographic process, the surface of the active matrix board is exposed in blocks. This is the reason why, if a deviation dx occurs in the alignment, the amount of overlap of the pixel electrode on the signal line differs from block to block and the transmissivity differs among the blocks of the active matrix type liquid crystal display apparatus. FIG. 32 shows the relationship between the alignment deviation dx and the difference xcex94T in transmissivity between a block having the alignment deviation dx and a block having no alignment deviation.
As is obvious, if the active matrix type liquid crystal display apparatus in which the pixel electrodes overlap the signal lines is driven by the dot inversion driving method, the amount of change in pixel electric potential caused by the coupling capacitances Csd really decreases, but differs largely among the photo-blocks. Consequently, there rises a big difference in the transmissivity among the blocks, leading to a problem called xe2x80x9cblock separationxe2x80x9d. As the size of the active matrix type liquid crystal display apparatus becomes larger, the number of blocks tends to increase more and more in the photolithographic process. Thus, there is a growing demand for suppression of the occurrence of the xe2x80x9cblock separationxe2x80x9d caused by the coupling capacitance Csd.
Therefore, it is an object of the present invention to provide an active matrix type liquid crystal display apparatus capable of preventing the image quality from deterioration due to a coupling capacitance between a signal line and a pixel electrode as well as suppressing the xe2x80x9cblock separationxe2x80x9d due to variations in the coupling capacitance.
In order to accomplish the above object, there is provided, according to an aspect of the present invention, an active matrix type liquid crystal display apparatus comprising:
an insulation substrate;
scanning lines formed on the insulation substrate;
signal lines extending in a direction intersecting a direction in which the scanning lines extend;
switching devices provided in the vicinity of each intersection of the scanning and signal lines such that the switching devices are arrayed in a matrix form;
an interlaminar insulation film disposed on or above the scanning lines, the signal lines, and the switching devices; and
pixel electrodes formed on the interlaminar insulation film and arranged in a matrix form, each electrode being connected to an output terminal of an associated switching device,
wherein only a part of each of opposite side portions of one pixel electrode widthwise covers two signal lines extending adjacent to the pixel electrode.
With the above arrangement, the two signal lines adjacent to one pixel electrode are widthwise covered only by a part of each side portion extending along the signal lines of the pixel electrode. Accordingly, there is no great change in the difference between a first capacitance between the pixel electrode and one of the two signal lines and a second capacitance between the pixel electrode and the other signal line even though there is a misalignment between layers. As a result, the so-called xe2x80x9cblock separationxe2x80x9d is suppressed, which would otherwise occur in a production process step in which photolithography is performed block by block.
In one embodiment, each pixel electrode covers the associated switching device. In the construction, it is possible to form the pixel electrode almost rectangularly, which leads to an increase in the area of the pixel electrode. Thus, the power consumption can be suppressed.
In one embodiment, each signal line is bent twice between two adjacent scanning lines such that two generally parallel but longitudinally displaced parts are formed, and these two parts are covered by opposed side portions of two adjacent pixel electrodes.
With this arrangement, the pixel electrodes can be formed in a rectangular shape. Thus, it is easy to form a color filter and/or a black matrix to be disposed on an opposed board confronting the insulation substrate, wherein the color filter may have a configuration similar to that of the pixel electrode and the black matrix may be formed so as to span a gap between two adjacent pixel electrodes.
In another embodiment, the parts of the pixel electrode covering the two signal lines adjacent to the pixel electrode are parts that overhang from side edges of the pixel electrode.
With the arrangement, the signal lines can be straight, and not bent, resulting in the reduced length of the signal line. This eventually prevents the delay of a source signal and/or the breaking or discontinuity of the signal line in a large active matrix type liquid crystal display apparatus having a size more than 15 inches.
Both side edges of each pixel electrode may be bent twice such that the overhanging parts are diagonally formed on the respective sides of the pixel electrode and these overhanging parts cover the two signal lines adjacent to the pixel electrode.
Each switching device may be disposed in the vicinity of a gap between two adjacent pixel electrodes. The gap between the two adjacent pixel electrodes and the vicinity thereof are regions that essentially should be shielded from light. Accordingly, it is unnecessary to dispose a black matrix dedicated to the switching elements. Thus, it is possible to prevent the increase of the area of the black matrix. Therefore, it is possible to obtain a large aperture ratio.
In one embodiment, the active matrix type liquid crystal display apparatus of this invention comprises an opposed board having a black matrix, and the black matrix is located between two adjacent pixel electrodes in such a manner that the black matrix overlaps each one of these pixel electrodes by at least an amount corresponding to an alignment margin of the opposed board relative to the insulation board.
In this case, positions of both side edges of the black matrix to be disposed on the opposed board are set taking the alignment margin between the opposed board and the insulation substrate into consideration. Accordingly, even though there is a misalignment between both substrates, the gap between the adjacent pixel electrodes is surely shielded from light and thus the occurrence of the so-called xe2x80x9cblock separationxe2x80x9d is suppressed to a higher extent.
In a location where the signal line is covered by the pixel electrode, an edge of the black matrix may be disposed along a center line of the pixel electrode or on an inner side of the pixel electrode than the center line.
A light shield film may be provided on the insulation substrate in such a manner that the light shield film spans a gap between the adjacent pixel electrodes.
Generally, the accuracy of alignment between the insulation substrate formed with the pixel electrodes and the opposed board confronting the insulation substrate is about xc2x15 xcexcm, whereas the accuracy of alignment between layers on the insulation substrate is less than xc2x11 xcexcm. Thus, the width of the light shield film is allowed to be smaller than that of the black matrix which would be disposed on the opposed board so as to span the gap between the adjacent pixel electrodes if no such light shield film was provided on the insulation substrate. In addition, it is not necessary any more to dispose on the opposed board a black matrix spanning the gap between the pixel electrodes. Consequently, the aperture ratio will be increased. Furthermore, because the total area of the black matrix disposed on the opposed board is reduced, it is possible to widen the bonding margin between the insulation substrate and the opposed board.
According to another aspect of the present invention, there is provided an active matrix type liquid crystal display apparatus comprising:
an insulation substrate;
scanning lines formed on the insulation substrate;
auxiliary capacitor lines arranged parallel to the scanning lines;
signal lines extending in a direction intersecting a direction in which the scanning lines extend;
switching devices provided in the vicinity of each intersection of the scanning and signal lines such that the switching devices are arrayed in a matrix form;
an interlaminar insulation film disposed on or above the scanning lines, auxiliary capacitor lines, the signal lines, and the switching devices; and
pixel electrodes formed on the interlaminar insulation film and arranged in a matrix form, each electrode being connected to an output terminal of the associated switching device,
wherein both side edges of each pixel electrode are bent twice such that the overhanging parts are formed on the respective sides of the pixel electrode and these overhanging parts cover two signal lines adjacent to the pixel electrode, and
wherein the auxiliary capacitor line underlies a portion between the two bents of each side edge of the pixel electrode.
With the above arrangement, the signal lines adjacent to the pixel electrode are widthwise covered by the respective overhanging parts of the pixel electrode. Therefore, a difference between a first capacitance between the pixel electrode and one of the two adjacent signal lines and a second capacitance between the pixel electrode and the other adjacent signal line is reduced. Thus, the shadowing phenomenon can be considerably suppressed by performing the dot inversion driving scheme.
The auxiliary capacitor line underlies the portion (referred to also as a xe2x80x9cbent portionxe2x80x9d) between the two bents of each side edge of the pixel electrode. Thus, the capacitance between the signal line and the pixel electrode at its bent portion is reduced. Consequently, the change in the coupling capacitance between the pixel electrode at the bent portion and the signal line due to a misalignment between layers is considerably reduced. Accordingly, it is possible to suppress the occurrence of the xe2x80x9cblock separationxe2x80x9d, which otherwise would occur in performing a lithographic process from block to block.
The auxiliary capacitor line may include electrode portions that extend toward a portion between the two bents of each side edge of the pixel electrode such that the electrode portions underlie the portions between the two bents of each side edge of the pixel electrode. In this case, the auxiliary capacitor line proper, namely a portion running parallel to the scanning lines of the auxiliary capacitor line can be located in any desired positions relative to the longitudinal direction of the pixel electrode.
A light shield film may be provided on the insulation substrate in such a manner that the light shield film spans a gap between the adjacent pixel electrodes.
In one embodiment, the light shield film is electrically connected to either the auxiliary capacitor line or the scanning line. In this case, owing to the field shield effect of the light shield film, a part of a line of electric force emitted from the signal line terminates at the auxiliary capacitor line or the scanning line. Thus, a first capacitance between the pixel electrode and one of the two adjacent signal lines and a second capacitance between the pixel electrode and the other adjacent signal line are reduced. As a result, the shadowing phenomenon due to the difference between the first and second capacitances is further suppressed, and the xe2x80x9cblock separationxe2x80x9d is well prevented from occurrence.
Instead of bending both side edges of each pixel electrode, each signal line may be bent. In this case, the signal line is bent twice between two adjacent scanning lines such that two generally parallel but longitudinally displaced parts are formed, and these two parts are covered by opposed side portions of two adjacent pixel electrodes, and the auxiliary capacitor line is located in a position corresponding to the portion between the two bents of the signal line. Similar effects can be achieved also in this case.
Other objects, features and advantages of the present invention will be obvious from the following description.